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CHAPTER 4
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How molecules are symbolized?
Cl Cl Cl2 Molecules may also have brackets to indicate numbers of atoms. E.g. Ca(OH)2 How many of each atom are in the following? a) NaOH b) Ca(OH)2 c) 3CaCl2 Na = 1, O = 1, H = 1 Ca = 1, O = 2, H = 2 Ca = 3, Cl = 6
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Law of Conservation of Mass
The law of conservation of mass states that matter can neither be created or destroyed during a chemical reaction. (Antoine Lavoisier said this!) CH4 + 2O2 CO2 + 2H2O 8 g g x g g The mass of CO2 needed is 22 g. **(the mass is the same on the right side and left side)
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Physical changes in matter 404
A Physical change is a change in the shape and appearance of matter. It is reversible. Tearing Cutting Folding Phase Changes* Mixing Dissolving
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Chemical changes in matter
A chemical change changes the nature and characteristic properties of matter. It is not reversible. Burning Rusting Cooking
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There are clues that a chemical change is occurring:
Signs of chemical change: Bubbles are forming Precipitate forms Color change Heat or light is given off Electricity produced
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Types of Chemical Changes (p117)
There are different types of changes and in chapter 4 we will study some of them: acid- base neutralization (404) combustion (404) cellular respiration (404) photosynthesis (404) synthesis (466) decomposition (466) precipitation (466) oxidation (466)
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Neutralization (404) Neutralization is when an acid and a base combine to produce salt and water. Example 1: HCl + NaOH --> NaCl +H2O (acid) + (base) --> (salt) + (water) Example 2: Acid rain is causing the acidification of Quebec’s lakes & soil. Lime (Ca(OH)2) is a base and can be added to lakes and soil to neutralize the acid and bring the pH of the lake or soil back to 7.
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Combustion (404) p. 121 Combustion releases a large amount of energy.
You need 3 things for combustion: a fuel (wood or propane) an oxidizing agent (air, oxygen) a certain temperature
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There are 3 types of combustion:
Rapid combustion: it releases a great deal of energy in a short period of time (a log fire, a burning candle) Spontaneous combustion: It is often unpredictable and could be disastrous. (on a very hot and dry summer day, wood can catch fire spontaneously - forest fires) Slow combustion: Occurs over a long period of time. (fermentation and cellular respiration)
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Cellular Respiration (404)
It is a slow combustion It takes place in our cells. In humans, it keeps our body temperatures at around 37oC. Glucose(sugar) and oxygen are used to generate energy. It produces carbon dioxide and water.
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EQUATION FOR RESPIRATION
CARBON DIOXIDE GLUCOSE C6H12O6 + 6O2 6CO2 + 6H2O + ENERGY OXYGEN WATER
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PHOTOSYNTHESIS (404) Plants make their own food
Through photosynthesis, plants use solar energy to make glucose (sugar) and oxygen from carbon dioxide and water. Without photosynthesis, life would not exist on Earth.
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EQUATION FOR PHOTOSYNTHESIS
WATER OXYGEN 6 CO2 + 6 H2O + ENERGY C6H12O6 + 6 O2 CARBON DIOXIDE GLUCOSE
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General Equation: A + B AB
Synthesis (466) p.118 C + O CO2 O C + O C O C O C O C O C O C O C O C O C O C O C O C O C O C O C General Equation: A + B AB
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General Equation: AB A + B
Decomposition (466) Cl Na Cl + Na General Equation: AB A + B
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Precipitation (466) p.118 When you combine two solutions and a solid is formed. The solid is called a precipitate. On page 118 of your textbook, you can find out how you can predict whether combining two liquids will give a precipitate.
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Oxidation (466) p. 120 Oxidation is a chemical change involving oxygen or a substance with properties similar to those of oxygen. Ex: rust forming
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Balancing Chemical Equations
Atoms are neither created or destroyed, only rearranged in a chemical reaction. The number of a particular atom is the same on both sides of a chemical equation Example: Mg + O2 MgO O Mg + Mg O However, this is not balanced! Left: Mg = 1, O = 2 Right: Mg = 1, O = 1
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EXOTHERMIC and ENDOTHERMIC Reactions 466 (p114)
Exothermic Reactions These chemical changes give off heat energy. C(s) + O2(g) CO2 (g) kJ Endothermic Reactions These reactions take in heat energy from their surroundings. N2 (g) + O2 (g) kJ NO (g)
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Endothermic = absorbing energy
Exothermic = releasing energy Law of conservation of energy = release and absorption of energy must be equal
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reaction Exothermic reaction, heat is given off & temperature of the water rises reaction Endothermic reaction, heat is taken in & temperature of the water drops
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Is it ENDO or EXO? Indicate if the following are exothermic or endothermic? A) gasoline burning exothermic B) a lake of freezing c) clothes drying on the line endothermic
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Exothermic energy level diagram
DH (‘delta H’) is the symbol for the ‘change in energy’. In an exothermic reaction the products have less energy than the reactants. DH is negative for an exothermic reaction.
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Endothermic energy level diagram
In an endothermic reaction the products have more energy than the reactants. DH is positive for an endothermic reaction.
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Making and breaking bonds.
(466)
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DH = H bonds broken - H bonds formed (reactants) (products)
Each type of chemical bond has a characteristic bond energy. Breaking bonds requires energy. It is endothermic. Making new bonds gives out energy. It is exothermic. The energy associated for different bonds can be found on page 115 of your textbook. To calculate DH use the formula: DH = H bonds broken - H bonds formed (reactants) (products)
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Single bonds energies (kJ/mol)
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Double and triple bonds energies (kJ/mol)
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H2(g) + Br2(g) --> 2HBr(g)
Calculate the DH for the above reaction. You need to use the values of page 115 of your textbook. DH = H bonds broken - H bonds formed DH =( ) - ( ) DH = kJ/mol Bonds Broken: H-H kJ/mol Br-Br 192 kJ/mol Bonds Formed: H-Br kJ/mol
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Radioactivity (466) p126 Radioactivity is a natural process where an unstable atom spontaneously transforms into a stable atom and in the process releases energy in the form of radiation. Henri Becquerel discovered the first radioactive element, Uranium. (1896) There are 3 types of radioactivity: Phosphorescent zinc sulfide detection screen + – Strong magnetic or electrostatic field Lead block Radioactive substance
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Types of Radiation (466) Alpha () particles are deflected towards the negative pole because they are positive particles. They can be stopped with a sheet of paper. Beta () particles are deflected towards the positive pole because they are negative particles. A sheet of foil 4mm thick can stop them. Gamma (g) rays are neutral. They have the greatest penetrating power. A very dense material (like lead) can stop them.
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Radiation Protection
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Uses of Radioactivity(466)
Treating cancer (kills cancer cells) Internal inspection Smoke detectors Sterilisation Food irradiation (to increase the shelf life of food)
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Radioactive Decay (466) Radioactive elements are unstable. They decay, change, into different elements over time. Here are some facts to remember: The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter how much you start with. After 1 half-life, half of it will have decayed. Each element has it’s own half-life ( page 129) The half-life of each element is constant. It’s like a clock keeping perfect time. Now let’s see how we can use half-life to determine the age of a rock or other artifact.
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The half-life of this element is 1 million years.
Questions may involve graphs like this one. The most common questions are: "What is the half-life of this element?" **Just remember that at the end of one half-life, 50% of the element will remain. Find 50% on the vertical axis, Follow the blue line over to the red curve and drop straight down to find the answer: Answer: The half-life of this element is 1 million years.
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After 2 million years 25% of the original material will remain.
Another common question is: "What percent of the material originally present will remain after 2 million years?" Find 2 million years on the bottom, horizontal axis. Then follow the green line up to the red curve. Go to the left and find the answer. Answer: After 2 million years 25% of the original material will remain.
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Nuclear reactions involve protons and neutrons in the nucleus.
Nuclear Reactions p Nuclear reactions involve protons and neutrons in the nucleus. During a nuclear reaction, the nucleus can lose or gain protons and neutrons. Compared to chemical reactions, nuclear reactions involve enormous energy change.
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DID YOU KNOW? Albert Einstein was the first scientist to realize the enormous amount of potential energy that is available in matter. He realized that mass and energy are related by the following equation: E=mc2 The c represents the speed of light (3.00x 108 m/s), E is energy and m is mass. Since c is so big, then even a small mass can be converted into an enormous amount of energy. This explain how a nuclear weapon, like the atomic bomb is so much more powerful than a chemical reaction like a bomb made with dynamite.
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Nuclear Fission Nuclear fission can be induced by bombarding atoms with neutrons. (discover by Otto Hahn, a German chemist) The nuclei of the atom is split into 2 or more lighter atomic nuclei. (fission means to split) Nuclear fission is the nuclear reaction that is most commonly used by humans.
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Nuclear Fission with Uranium
235U n Ba Kr + 3 1n + Energy
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The Fission Process p. 130 A neutron travels at high speed towards a uranium-235 nucleus. U 235 92 n 1
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The Fission Process A neutron travels at high speed towards a uranium-235 nucleus. U 235 92 n 1
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The Fission Process A neutron travels at high speed towards a uranium-235 nucleus. U 235 92 n 1
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The Fission Process The neutron strikes the nucleus which then captures the neutron. U 235 92 n 1
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The Fission Process The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron. U 236 92
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The Fission Process The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
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The Fission Process The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
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The Fission Process The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
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The Fission Process It then splits into 2 fission fragments and releases neutrons. n 1 141 56 Ba n 1 92 36 Kr n 1
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The Fission Process It then splits into 2 fission fragments and releases neutrons. n 1 141 56 Ba n 1 92 36 Kr n 1
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The Fission Process It then splits into 2 fission fragments and releases neutrons. n 1 141 56 Ba n 1 92 36 Kr n 1
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The Fission Process It then splits into 2 fission fragments and releases neutrons. n 1 141 56 Ba n 1 92 36 Kr n 1
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U + Ba n 3 Kr U + Cs n 2 Rb Nuclear Fission Examples 235 92 141 56 1
3 Kr 36 U 235 92 + Cs 138 55 n 1 2 Rb 96 37
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H + He n Energy Nuclear Fusion p.131 2 1 4 3
In nuclear fusion, two nuclei with low mass numbers combine to produce a single nucleus with a higher mass number. The most common type of fusion reaction is the reaction in the sun, hydrogen nuclei fuse to form helium. H 2 1 + He 4 n 3 Energy
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Nuclear Fusion Fusion small nuclei combine 2H + 3H 4He + 1n + 1 1 2 0
Occurs in the sun and other stars Energy
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The Fusion Process H 2 1 H 3 1
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The Fusion Process H 2 1 H 3 1
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The Fusion Process H 2 1 H 3 1
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The Fusion Process H 2 1 H 3 1
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The Fusion Process
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The Fusion Process
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The Fusion Process
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The Fusion Process
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The Fusion Process n 1 He 4 2 ENERGY
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The Fusion Process n 1 He 4 2 ENERGY
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The Fusion Process n 1 He 4 2 ENERGY
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The Fusion Process n 1 He 4 2 ENERGY
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